1. Field of the Invention
The present invention relates to a development device in which a two-component developer is used and an image forming apparatus including the same.
2. Description of the Related Art
Recently a two-component developer (hereinafter simply referred to as “developer”) having excellent toner charging stability is widely used in an electrophotographic image forming apparatus that meets a full-color image and a high-quality image. The developer includes a toner and a carrier. When the developer is agitated in a developer tank of a development device, a properly-charged toner is obtained by friction between the toner and the carrier. In the development device, the charged toner is supplied to a surface of a development roller, and the charged toner is moved to an electrostatic latent image formed on a photoconductive drum from the development roller by an electrostatic suction power. Therefore, a toner image is formed on the photoconductive drum based on the electrostatic latent image.
Additionally, there is a demand for a high-speed, miniaturized image forming apparatus. Therefore, it is necessary to quickly and sufficiently charge the developer, and also to quickly transfer the developer. For example, Japanese Patent Application Laid-Open No. 2001-255723 proposes a cycling type development device including first and second developer conveyance paths that are divided by a partition plate provided in the developer tank, first and second communication paths that communicate the first developer conveyance path and the second developer conveyance path on both end sides, and first and second auger screws that are disposed in the first and second developer conveyance paths to mutually convey the developer in opposite directions.
Because the development device of Japanese Patent Application Laid-Open No. 2001-255723 does not include a driving section that conveys the developer in the first and second communication paths, a flow of the developer tends to be disrupted in the first and second communication paths. On the other hand, Japanese Patent Application Laid-Open No. 4-240677 proposes a development device in which third and fourth auger screws are provided in a direction orthogonal to the first and second auger screws in the first and second communication paths.
However, there is a problem in the development device of Japanese Patent Application Laid-Open No. 4-240677. The first auger screw is disposed from an upstream end to a downstream end in the first developer conveyance path, and the second auger screw is disposed from the upstream end to the downstream end in the second developer conveyance paths. Therefore, the third and fourth auger screws in the communication paths and the first and second auger screws are designed so as not to come into contact with each other. That is, in an intersection part in which each developer conveyance path and each communication path intersect at right angle, a rotating shaft of the first and second auger screws differs from a rotating shaft of the third and fourth auger screws in a height, and a spiral blade of each auger screw is eliminated. Therefore, in the development device of Japanese Patent Application Laid-Open No. 4-240677, the developer is easily retained in four corners at the upstream end and the downstream end of the first and second developer conveyance paths.
A compressive force is applied to the developer retained in the four corners of the developer tank by a developer conveyed by each auger screw. When such stress as the compressive force continues over a long period, a fluidity improver that is of a toner additive is buried in resin particles constituting the toner, which results in a phenomenon in which developer fluidity is extremely degraded to hardly convey the developer. As a result, a sufficient amount of the developer is hardly supplied to the photoconductive drum through the development roller, which results in a problem in that density of the image printed on a recording medium is reduced.